The present disclosure relates, in general, to electronics, and more particularly, to circuits and methods for controlling power to electronic devices that emit visible light, such as Light Emitting Diodes (LEDs), such as power produced from an Alternating Current (AC) power source.
Single-stage LED drivers use a single Switched Mode Power Supply (SMPS) circuit to convert an AC mains line voltage to a Direct Current (DC) voltage for one or more LEDs. The SMPS circuit may also provide Power Factor Correction (PFC). These SMPS circuit are typically designed for simplicity and low system cost.
One problem encountered by single-stage LED drivers is AC mains line ripple in the LED current. Even when the magnitude of the ripple on the voltage provided to the LEDs is small, the LED current ripple can be large due to the exponential V-I curve of an LED load.
The LED ripple current caused a corresponding variation in the magnitude of the light emitted by the LEDs. Even small fluctuations in the magnitude of illumination in a frequency range such as, for example, 5 Hz to 2000 Hz can be uncomfortable and/or harmful to the human eye. For the safety and comfort of the human eye, in the frequency range of 100 to 120 Hz at which the ripple current occurs (the second harmonic of the AC mains frequency), the magnitude of the LED current ripple should be less than, for example, 2% of the total LED current.
Technologies for reducing the LED ripple current of the related arts include using a second stage in the LED driver, wherein the second stage is a DC-to-DC SMPS circuit. However, adding a second SMPS circuit increases the Bill-Of-Materials (BOM) cost of the LED driver.
Other technologies for reducing the LED ripple current of the related arts include using constant-current a Low Drop Out (LDO) regulators in series with the LEDs. This is less expensive than adding a second SMPS circuit, but reduces the total system efficiency because of the headroom loss across the LDO regulator.
It would be advantageous to reduce the effect on the human eye of LED ripple currents in the 100-120 Hz range at a lower BOM cost than when using a second-stage SMPS circuit and at higher efficiencies than possible using a second stage LDO regulator.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of the embodiments.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments. This avoids obscuring the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the disclosures herein. The details of well-known elements, structures, or processes that are necessary to practice the embodiments and that are well known to those of skill in the art may not be shown and should be assumed present unless otherwise indicated.